1. Field of the Invention
The present invention relates to an optoelectronic material, its application device and a method of manufacturing the optoelectronic material. More particularly, this invention relates to an optoelectronic material that comprises, as the core, semiconductor ultrafine particles with controlled particle sizes, formed of a material whose quantity is unlimited and which is free of environmental contamination, and that is excellent in silicon (Si)-LSI technology matching, spontaneous light emission, fast response, pixel miniaturization, low dissipation power, high environmental resistance and assemblyless process, an application device of the optoelectronic material, and a method of manufacturing the optoelectronic material.
2. Description of the Related Art
Conventional light-emitting devices include a light-emitting diode and devices utilizing electroluminescence which have been put to a practical use. Optoelectronic materials used for those light-emitting devices are compound semiconductors essentially containing a group III element and a group V element (hereinafter referred to as “group III-V”) in a periodic table or compound semiconductors essentially containing a group II element and a group VI element in the periodic table, not silicon (Si). This is because silicon is an indirect transition semiconductor and the band gap, 1.1 eV, lies in a near infrared region, which has been considered as impossible to realize a light-emitting device in a visible light region.
Since the observation of visible light emission of porous Si at room temperature in 1990 (disclosed in, for example, L. T. Canham, Applied Physics letters Vol. 57, No. 10, 1046 (1990)). Enthusiastic studies have been done on the characteristics of visible light emission at room temperature with Si as a base material. Most of those reports are concerned with porous Si.
This luminous porous Si is basically formed by anodization of the surface of a single crystalline Si substrate in a solution essentially containing hydrogen fluoride, and photoluminescences (PL) of several wavelengths in a range from 800 nm (red) to 425 nm (blue) have been observed up to now. Recently, attempts have been made to luminescence by current injection excitation (electroluminescence; EL) (e.g., Published Unexamined Application No. 5-206514).
EL of those porous Si have the following characteristic properties. (1) The spectra of EL and PL show substantially the same shapes with some difference in intensity though. (2) The EL intensity is proportional to the injection current in a supposedly practically usable range of the injection current density. It is to be noted however that in a range where the injection current density is lower than the former range, it has been reported that the EL intensity is proportional to the square of the injection current.
The property (1) indicates that the EL and PL are caused by the recombination of carriers (excited electron-hole pairs) through approximately same luminescence levels, and the property (2) indicates that the generation of carriers essential to EL is mostly accomplished by the injection of minor carriers in the vicinity of the p-n junction.
With regard to the emission mechanism of Si which is an indirect transition semiconductor, there are a view that the wave number selection rule for optical transition is relaxed in a three-dimensional minute structural area of the nanometer (nm) order in the porous shape, thus ensuring the radiative recombination of electron-hole pairs, and a view that a many-remembered ring oxide (polysiloxene) is formed on the surface of porous Si and new energy level which contributes to the radiative recombination is formed at the polysiloxene/Si interface. In any case, it seems certain that with regard to photo excitation, a change in energy band structure (a phenomenon of increasing the gap energy) is caused by quantum confinement effect.
Further, luminescence from porous Si has a broad spectrum width of approximately 0.3 eV or wider. In this respect, some attempts have been made to form a cavity structure using this porous Si in order to enhance the intensity of a specific wavelength region in the continuous spectrum that is originally generated (e.g., L. Pavesi et al., Applied Physics Letters Vol. 67, 3280 (1995)).
Because the conventional optoelectronic materials use compound semiconductors mainly consisting a group III-V element or a group II-VI element of a direct transition type, however, they contain an element (In or the like) whose quantity is significantly small and whose refining cost is high while the emission efficiency is high. Further, a fine patterning for those compound semiconductors as a semiconductor fabrication technique is not ripe yet as compared with a fine patterning for Si, making it difficult to form a fine pattern of the micron (μm) order or smaller. Furthermore, group III and V elements serve as a dopant in Si, and thus affect the electrical conductivity. That is, while a spontaneous light-emitting device essentially consists of a semiconductor material, the matching with the process and device technologies for Si-LSI as a typical electronic device is poor and it is substantially impossible to fabricate a device with integrated Si and LSI. Moreover, there is an essential problem that the type of the material should be changed (i.e., it should newly be found) and the fabrication method should be reconstructed entirely in order to adjust the emission wavelength.
With regard to luminous porous Si, a porous layer is formed on the surface of a single crystalline Si substrate by anodization in a solution so that while a crystalline in the porous layer has an excellent crystallinity, it is difficult to control the shape and size of crystalline. It is particularly difficult to efficiently produce a spherical crystalline of 5 nm or less in particle size. If the mechanism of visible light emission of an Si-based group IV material is a quantum size effect (relaxation of the wave number selection rule, a change in band structure due to a quantum confinement effect, or the like), it is still essential to produce a spherical crystalline having a particle size of the nm order. In view of this, the fabrication technique cannot be said to be the optimum one.
A difficulty also arises when one intends to demonstrate the performance of a display device by regularly arranging porous Si based light-emitting elements and independently operating them. Specifically, since the porous Si is directly formed in an Si substrate, it is not possible to maintain the electric independence (insulation) between the elements. Further, it is not possible to form a lamination structure with another material like a transparent material having a high transmittance in the visible region.
Although a scheme of arranging particles of a group IV element or partly oxidized particles thereof between electrodes for light emission is disclosed (e.g., Published Examined Application No. 7-52670), it has a difficulty in controlling the electric characteristic and cannot be adapted to various kinds of light-emitting devices and photodetectors. Therefore, the state-of-the-art technology does not provide an optoelectronic material which can be adapted to various kinds of light-emitting devices and photodetectors by controlling the electric characteristics.